This invention relates to the ball bonding of aluminum bonding wire, particularly for use in the microelectronics field.
Very fine aluminum and gold based wires are commonly used to connect integrated circuits to lead frame packages. In a typical system,, the integrated circuit is placed in the package center and bonding wire segments connect pads on the integrated circuit perimeter to leads embedded in the package.
Bonding wire segments are typically joined to integrated circuits and lead frames by either the wedge-wedge bonding method or the ball-wedge method. In the ball-wedge method, the bonding wire is fed through a capillary hole in a bonding head until the tip of the wire protrudes a short distance. Then an electric arc melts the protruding wire tip into a ball. After arcing ceases, the molten ball quickly solidifies because heat is transferred into both the wire stem and the surrounding gas. The solid ball is pressed and flattened onto the contact pad by means of the bonding head and ultrasonic vibrations are applied to the ball-pad interface to form a bond (ball-bond) between the ball and the pad. The bonding head then threads the wire to the lead by a looping path and the wire under the bonding head is pressed onto the lead while ultrasonic vibrations form a bond (wedge-bond) between the wire and the lead. As the bonding head lifts away from the wedge-bond, the wire is pulled apart, leaving the wire tip protruding for the next ball-wedge bonding cycle.
Although aluminum wire has strength and cost advantages over gold wire, it is not used in ball-wedge bonding because it does not form a ball as well as gold wire. Specifically, the aluminum wire forms an asymmetric, porous, faceted ball that ball-bonds very poorly, while the gold wire forms a round, axisymmetric ball that ball-bonds consistently well.
Some researchers, such as Gehman, B. L., Ritala, K. E. and L. C. Erickson, "Aluminum Wire For Thermosonic Ball Bonding in Semiconductor Devices", Solid State Technology, October 1983, pp. 151-158, have blamed the poor ball-bonding behaviour of aluminum wire on the oxide film present on the wire. For instance, in air at ambient temperature an oxide film having a thickness of about 0.005 microns is formed. Gehman, et al have shown that the oxide film is strong enough to prevent surface tension from forming a ball for some time after melting of the aluminum wire tip. It has been suggested that further heating by the arc breaks up the oxide sheath by partially melting it or by boiling components of the cylindrical, molten wire. The molten wire then contracts to the most compact geometry possible with the oxide fragments left on the molten metal surface.
Other investigations have shown that the arc entry location into the wire shifts about to new locations of reduced resistance on the aluminum wire surface causing these local areas to melt and oxidize. Both oxidation and melting of aluminum at the arc entry area contribute to shifting arc entry location because both increase the electrical resistance of the initial entry area, forcing the arc up the wire to areas of lower resistance. Shifting of the arc also causes the partially molten wire tip to bend and twitch in response to the changing electromagnetic and surface forces.
Thus, it would appear that irregular balls on aluminum wire result from (1) shifting arc entry into the aluminum wire, (2) localized oxide melt and/or metal boiling required to break-up the oxide and (3) persistance of oxide fragments on the collapsed, molten wire tip during solidification.
It is the object of the present invention to provide a method for improving the ball-forming behaviour of aluminum bonding wire.